U.S. Pat. No. 5,190,403 to the present inventor provides the following introduction to the problem of water erosion:
The interface of land and water presents serious erosion, or land loss problems. In particular, where waves impact land structures, such as beaches or promontories, the wave energy can disturb the land structure causing the land to erode into the water. The damage caused by this action can take years to accrue, such that the day to day or month to month change is imperceptible, or the damage can be seen in a matter of days where high water or unusually fierce storms generate very high waves.
Where the beach or land adjacent a body of water [or a watercourse] erodes, valuable real estate and improvements may be permanently lost, or the land may be rendered unsuitable for improvements. Ocean and lakefront property [and likewise riverside and even creek-front property] is very valuable. Thus, the constant erosion of land adjacent these waters is a costly problem. Further, where the erosion problem is caused by currents, waves or eddies undercutting banks, shorelines or mounting structures, serious damage to the structures adjacent the undercutting can occur. For example, where a jetty or pier is constructed outward into a body of water, currents and eddies can undermine the soil structure at the lake, river or seabed adjacent the footings or pilings which support the structure. The resulting erosion can undermine the integrity of the structure, requiring filling of the eroded area and repair of the structure and footings or pilings.
In order to address this problem, U.S. Pat. No. 5,190,403 describes a multi-legged erosion protection device designed to be used with other similar or identical devices to create a flexible and supportive matrix structure. The matrix can be formed by assembling the devices together randomly or alternatively in a uniform pattern. In either case the multi-legged configuration of the devices enables interlocking or “nesting” between the respective devices to form the matrix structure for reinforcing a water/land interface (e.g. beach, breakwater, river bank etc). The matrix structure created in this way can be dense, secure and flexible enough to absorb and reflect the energy of waves or water flows, thereby preventing the waves or water flows from dislodging or eroding the land protected/reinforced by the structure.
In addition to being dense and supportive as just described, the matrix structure of interlocking devices is also highly permeable with void spaces inside and extending through the matrix. These voids provide habitat for fish and other marine life when the structure is used as a reef or breakwater in river, lake and/or coastal applications. Alternatively, where the structure is applied to reinforce a riverbank or the like, the voids can allow back filling with soil to enable roots to penetrate and plants and vegetation to grow in/over/around the supporting structure to (re-)vegetate the bank.
The preferred form of the multi-legged device (armour unit) described in U.S. Pat. No. 5,190,403 is shown in FIG. 1 and is shaped like a jack (i.e. like one of the 6 pointed items used in the traditional game of “Jacks”). The armour units are typically made of concrete. The commonly produced sizes of armour units range from very small units with a volume of around 0.016 m3 and a weight of approximately 36 kg to very large units having a volume of approximately 2 m3 and a weight of approximately 4600 kg. Even larger units weighing up to 20 tonnes have been (and may continue to be) produced and used as well. The small units are typically used in applications such as stream bank restoration, whereas the larger units are generally used for energy dissipation and ocean applications.
FIG. 1 shows that each armour unit 1 is generally symmetrical and has six legs. In FIG. 1, some of the legs are designated 2a whilst others are designated 2b. The distinction between legs 2a and 2b will be explained below. All of the legs are approximately the same length and thickness, and they all have a generally square cross-section. In each corner formed by the intersection between a leg 2a and a leg 2b is a density spacer 3. Each density spacer 3 is a block which is normally integrally formed with the legs of the armour unit 1. The density spacers 3 function to space the placement of the legs of adjacent armour units when a plurality of the armour units are nested together as shown in FIG. 4. In FIG. 4, the units are arranged in a uniform pattern. It is also possible to create a matrix structure (suitable e.g. for use in breakwaters etc) by placing the units randomly as shown in FIG. 13. Some versions of the armour unit may have additional density spacers, for example 3a as shown in FIG. 3.
FIG. 2 illustrates that each armour unit 1 is formed from two halves 4. Each half 4 is generally T-shaped with one leg 2b forming the stem portion of the T and the other two legs 2a together forming the crossbar portion of the T. Both halves 4 are substantially identical. In each half 4, a recessed notch 5 is located in the centre of the top surface of the crossbar portion, between the two legs 2a. Each half 4, when made of concrete, is typically formed using a 2-D “cookie cutter” construction. This involves simply pouring concrete into a mould the shape of the half 4 and allowing the concrete to set.
The two-part construction of the preferred versions of the armour units has a number of benefits. Firstly, the “2-D” moulds required to form each of the halves 4 are much cheaper than the “3-D” moulds that would be required to form an armour unit 1 as a single piece. Also, a 3-D mould such as this could not be stripped for a considerable amount of time (perhaps several days for larger units) because the stripping of the mould could not take place until all of the concrete legs and their extremities became self-supporting. In contrast, it can take as little as a few hours after pouring for the concrete to set sufficiently to allow the 2-D moulds to be stripped leaving formed halves 4. Furthermore, multiple individual halves 4 can be much more conveniently stacked together for storage and transportation than could multiple one-piece “Jack” shaped units.
In order to assemble two halves 4 to form an armour unit 1, the halves are placed adjacent each other in a crosswise fashioned (as shown in FIG. 2) such that the legs 2b of each half are collinear and the crossbar portions of each half are disposed at right angles to each other. For small units, this can be done by hand, but as noted above, a crane is required to move the halves 4 of larger units. The halves 4 are then brought together as shown by the dashed lines in FIG. 2.
FIG. 3 shows a partially cross-sectional view of the unit 1 immediately after the two halves 4 have been brought together. In FIG. 3, the lower half 4 is shown in a plan orientation. However, the upper half 4 in FIG. 3 (if it were shown in full) would be oriented with the legs 2a pointing in and out of the page. Instead, the upper half 4 is shown in cross-sectional view with only the cross-section of its leg 2b visible.
From FIG. 3 it can be seen that when the two halves 4 are brought together, a small gap 6 exists between them. This gap exists to allow grout to be injected after the halves have been brought together to bond the halves together. The gap is normally at least 10 mm, this being the minimum space required to allow pourable (or pumpable) grout to be used.
Whilst the preferred version of the armour unit in U.S. Pat. No. 5,190,403 (which is described above with reference to FIGS. 1-4 and 13) has proven to be hugely effective in protecting and/or reinforcing beaches, river banks, breakwaters etc against water erosion, nevertheless certain problems have emerged, particularly in relation to its assembly and quality control.
One of the problems relates to the amount of grout required to bond the individual halves together. It will be appreciated that a large (or sometimes very large) quantity of grout can be required to fill the entire void 6, particularly for large armour units. This can significantly increase the material requirements (and hence the cost) of constructing the armour units. It can also significantly increase the time required to assemble the units as time is required to pour/pump the grout in and allow it to set—the more grout needed to be used, the more time is required.
A related problem arises where the units are assembled and used in poor/developing parts of the world. It will be appreciated that, even though the process of slotting the halves 4 together and pouring/pumping grout into the gap 6 is relatively simple, nevertheless it requires skilled execution as well as the use of high-quality grouting materials. In poor/developing parts of the world there is sometimes a tendency to use poorer quality materials in an effort to save cost. However, the use of poor quality grouting materials in this application jeopardises the integrity of the armour unit.
Another problem arises because, before they are bonded together by the grout, the individual halves 4 can be quite fragile. In particular, the manipulating and wiggling that often occurs when workers attempt to slot the two halves 4 together can cause outward bending and/or impact loads on the parallel sidewalls of the notches 5. These loads can induce stresses (including potentially damaging tensile stresses) in the concrete. This can be a problem particularly near the corners 7 identified in FIG. 3 which create regions of potentially high stress concentration. Consequently, with the configuration of the halves shown in FIGS. 2-3 there is an increased risk of fracture in and around the planes A-A due to the above-mentioned stress concentrations. This fragility reduces considerably when the grout sets to firmly bond the halves together forming a single unit with a solid core of grout and concrete at the centre.
It is an object of the invention to provide an improved erosion protection device which helps to address one or more of the above-mentioned problems, or which at least provides a useful or commercial alternative to existing devices in the marketplace.
It will be clearly appreciated that any reference herein to previous or conventional methods, devices, practices, or other information (including publications) does not constitute an acknowledgement or admission that any methods, devices, practices or other information (including publications), or any possible combination thereof, formed part of the common general knowledge in the field, or is otherwise admissible prior art, whether in Australia or in any other country.